Telephone system



July 14, 1936.

T. F. CROCKER 2,047,619

TELEPHONE SYSTEM Filed Dec. 29, 1954 4 Sheets-Sheet 2 FIG.1 PART 2 RINGLINE

HOLD

INVENTOR. THOMAS F CROCKER ATTORNEY.

Patented July 14, 1936 UNITED STATES PATENT orrics Associated ElectricLaboratories, Inc.,

Chicago,

111., a corporation of Delaware Application December 29, 1934, SerialNo. 759,643

14 Claims.

The present invention relates to telephone systems in general, but isconcerned more particularly with telephone systems including a mainexchange and a private branch exchange, and the object of the inventionis the production of a new and improved private branch exchange,

system by means of which service may be rendered more rapidly andefilciently, and by means of equipment of relatively low cost.

General description I contains desirable features as follows:

(1) Each trunk line extending between the branch exchange and the mainexchange terminates at the branch exchange in a switchboard cord andplug by means of which the operator may connect the trunk line with thejack of a local line to complete an incoming call or to extend anoutgoing call, while a transfer jack is associated with each trunk, sothat the operator can use a local double-plug cord and temporarilytransfer the local line from in connection with the trunk line intoconnection with another local line, while holding a condition ofanswered supervision on the trunk line.

(2) Each trunk cord is provided with a differential recall relay,operated from a grounding recall key at the subscriber's station, whichcontrols a pair of recall relays so that the local subscriber ofinter-exchange connections can control a recall lamp at will to signalthe operator. Relay equipment, controlled from the recall relay, bringsabout the lighting of the recall lamp upon the first manipulation of therecall button, together with the transfer of the local end of theconnection to the transfer jack, while on the next manipulation of therecall button the local connection made through the transfer jack by theoperator issevered and the original trunk connection reestablished. Thisfeature enables a branch exchange subscriber to be switched away from amain exchange connection temporarily and into connection with anotherbranch exchange subscriber to obtain additional information, forexample, following which the main exchange connection may be resumed.

(3) In order to better facilitate the work of the 5 branch exchangeoperator in directing incoming calls to the appropriate lines, accordingto the character of information desired for example, a so-called holdsignal or lamp is provided which becomes lighted when the operator firstthrows her talking key to answer an incoming call and remains lighteduntil the call has been disposed of, as without this signal there is atendency for the operator to restore her talking key and then go aboutprocuring information as to the disposition of an incoming call and thenhave her attention distracted so that she forgets that there is anundisposed-of call waiting on the trunk, as the usual call signal ispreferably extinguished when the operator responds. At any rate, the 29special signal referred to serves to indicate to the operator that shehas already conversed with the subscriber whose line is connected with atrunk in association with which the signal is displayed. By throwing hertrunk-key handle in the other direction, the operator may extinguish thespecial signal and return the trunk cord to normal condition in theevent that she herself is able to supply the desired information withoutextending the incoming call to a sub- 30 scribers line.

(4) In order to relieve the operator of the necessity of continuing tosignal the called local sub-station at intervals until a response isobtained, circuit arrangements are provided such that the operator maymanipulate a ring-start key in her position equipment and start theringing operation in the cord of which the talking key is thrown,following which the ringing current is automatically thrown andintermittently applied until the called subscriber responds, whereuponthe ringing current is automatically disconnected, and the talkingcircuit established, and answered supervision given.

(5) In order to expedite the taking down of connections, the circuitarrangements are provided so that the release of the line relay in thetrunk cord when the local subscriber replaces his receiver at the end ofa conversation opens the trunk as regards the inter-exchange conductorsthereof, giving disconnect supervision in the main exchange. In thisway, disconnect supervision is given immediately to the main exchange.

(6) In order to enable the branch exchange operator to use the trunkcord and plug over which a call has been received to set up a connectionto a local line tentatively or solely for the purpose of securinginformation as to the disposition of an incoming call, provisions aremade whereby the operator is able to answer an incoming call and thendisconnect the incoming trunk from the associated cord plug temporarilyby means of a so-called split relay and then use the plug and cord tomake her local inquiry connection while the calling subscriber istelephonically disconnected from the terminating cord and plug, and astate of answered supervision is maintained on the inter-exchange trunk,the split condition being terminated by the operator as soon as she hasobtained the desired information.

(7) In order to insure the secrecy of conversations against invasion oreaves dropping" by the branch-exchange operator, arrangements areprovided such that a warning tone-signal is transmitted over anestablished conversational circuit in the event that the operator throwsa talking key of a cord over which the conversation is taking place,this arrangement being controlled from the calling supervisory relaythrough contacts of the associated talking key.

(8) A further feature of the disclosed system is that the branchexchange lines are divided into two classes as regards the establishmentof inter-exchange connections. The lines of the unrestricted classterminate in the switchboard jacks of which the sleeve is directlygrounded, while the lines of the restricted class each terminate in aswitchboard jack having the sleeve element grounded through a resistorwhich so limits the current flow as to render the marginal sleeve relayof the trunk cord non-responsive. The circuit arrangement is such thatthe completion of a subscriber-to-subscriber talking circuit through atrunk cord is dependent upon the sleeve relay having been operated.

In connection with this feature, special provisions have been made forpreventing the operator from either deliberately or unintentionallythwarting the arrangement by operating a talking key of a local cord andsimultaneously a talking key of a trunk cord to permit conversationthrough her headset conductors from a restricted line to a trunk. Thisis accomplished by providing separate leads from the operators headsetto the local cord and to the trunk cord, in combination with adisconnect relay, operable whenever a talking key on a trunk cord isthrown, to disconnect the talking conductors of the local cord from theoperators headset and from the talking conductors of the trunk cord.

(9) Each of the inter-exchange trunk cords is provided with a key whichtransfers the trunk conductors to a pair of calling device leads, whenoperated. In the event that the branch exchange is operated inconjunction with a manual exchange no calling device is provided, inwhich case the said key may be used as a flash key, as it opens thetrunk conductors each time it is operated, in the event that no callingdevice is connected.

(10) The trunk cords are each arranged so that so-called night servicemay be given to one or more local lines. A night key, when operated,by-passes the repeating coil of the cord thereby extending the trunkconductors through to the plug of the cord, whereby the line into whichthe cord is plugged may be used to originate and receive calls directlyover the associated inter-exchange trunk. The usual operators signals atthe trunk cord so plugged up for the night service are disabled pursuantto the operation of the above-mentioned night key. 5

(11) It is contemplated that the private branch exchange systemdisclosed herein be used in business establishments and the like whereina night watchman, or other person whose principal duty is somethingother than to operate the switchboard, is on the premises at times otherthan during regular working hours, and whose auxiliary duty is tooperate the switchboard whenever there is need for so doing. Therefore,an arrangement for sounding a night alarm has been provided whichrequires no special design of the regular equipment. This night alarm isset oil! responsive to a momentary impulse occasioned by a change insignal current which occurs when a line lamp is lighted. 20

Description of drawings Referring now to the accompanying drawings,comprising Figs. 1 to 5, they show, by means of the usual circuitdiagrams a sufiicient amount of 25 equipment in a system embodying theprinciples of the invention to enable the invention to be understood.More in particular, Figs. 1 to 4 are circuit drawings illustratingcircuits used in a private branch exchange having a non-multipleswitchboard, while Fig. 5 is a switching diagram which indicates thearrangement employed for interconnecting the lines at the switchboard.

Referring now particularly to Fig. 5, three local substations (A, B, andC) are shown interconnected with the switchboard, whereat they terminatein jacks J I, J 2, and J3. At the switchboard, a number of local cordssuch as LC are provided, the cord LC having plugs P2 and P3,

which may be inserted into the jacks for the purpose of interconnectinga pair of lines.

The trunk line TL is one of a desired number of two-way trunk lineswhich extend from the illustrated branch exchange to the main exchange(not shown). the trunk line TL terminates in the trunk cord TC and theplug Pl, which may be inserted into any of the jacks.

The trunk TL is provided also with a transfer jack J 4, which is used inconnection with a local cord circuit such as LC to interconnect the plugPl with another local line when the local subscriber involved in aninter-exchange call wishes to converse with another local subscribertemporarily. 5

Referring now to Figs. 1 to 4, Fig. 1 shows the inter-exchange trunkline TL, terminating at the branch exchange in the trunk cord TC and theplug Pl, together with the line of substation A and the terminating jackJ I;

Fig. 2 shows the local cord LC, interconnecting the plugs P2 and P3,together with the lines of substations B and C and the associatedswitchboard jacks J 2 and J3;

3 shows the pe ator's headset and posi- (H At the branch exchange,

. of contacts of ring-start relay 2|2.

relay 2 operates through registor 201 and ground conductor 25|,whereupon calling supertion with the tone lead. 300, as will be pointedout hereinafter; and v Fig. 4 shows the night alarm circuit includingthe induction coil 10, thenight alarm key K1,

relays 40| to 403, and the impedance coil 404 and,

night alarm buzzer 405. The night alarm circuit is associated with thecord-supervisory and linesignals by way of current-supply conductor M2,and with the local line circuits by way of'currentsupply conductor 4| I.It is to be noted that conductor 4' is normally connected to theungrounded negative pole of the exchange battery by way of the uppercontacts of the night alarm key, and that the negative pole of theexchange .pf the conductors 3| |-3|6 of the multiple cable ,DETAILEDDESCRIPTION The disclosure having been described generally, a detaileddescription of the operation of the exchange equipment will now begiven, in which the foregoing and additional features willbecomeapparent. It will be explained first how a local call is set up inthe private branch exchange.

Local call Assuming now that the restricted-line subscriber atsubstation C desires to converse with the subscriber at substation A,for example, the subscriber at substation-C removes his receiver, (notshown) thereby closing the usual circuit over line conductors 205.and206 for line lamp 208 by way of lamp conductor 4| Upon noting thelighted condition of the lamp 208, the branch-exchange operator insertsthe answering plug of an idle'local cord circuit, the plug P2 of thelocal cord LC, for example, into the jack'J3. The cut-off contacts ofthe jack J3 clear the calling line of its ground and lamp connections. Acircuit is now closed over the sleeve circuit by way of the sleeveresistor 201 for the upper winding of ring cut ofi relay 2| by wayRing-cut-ofi visory lamp 2|! becomes lighted as a supervisory signal,but the answering'supervisory relay 2|6 does not become lighted atthis'time because its' circuit is,opened at the contacts of theanswering bridge relay 209, which relay is now energized- \groundpotential directly to the transmitterpotential to the conductor 30lnergizes the transmitter Tby way of the windings of impedance coil 3 I8and thelower left-hand winding of the induction coil 10. This provisionis made so as to economize on the flow of current; it is unneces sary tohave the transmitter energized except during conversation. Since thecontacts of the key which energize the transmitter close first, theclick which would otherwise be transmitted to the calling line isdissipated by the time the talking conductors are connected up. It maybe noted that the shunt condenser -3|9 at the operators headset forms'analternating-current bypass for the impedance coil 3|8, thus localizingthe voice currents generated at the transmitter T, and, in cooperationwith the impedance coil 3|8, efiectively prevents flow of voice currentsin the current-supply leads connected to the impedance coil 3|8.

The talking key K4 of the cord LC also associates the common ring-startconductor 303 with the ring-start relay 2|2 in the cord circuit; it

connects the tone-relay conductor 304 to the contacts of the callingbridge relay 2|0, both for a purpose which will be explainedsubsequently.

When the operator converses with the sub scriber at substation C andlearns that he desires to be connected with the line of substation A,she is able at once to determine whether or not the line of substation Ais engaged by merely looking to see whether or not there is a plug inthe jack J I, as this is a non-multiple switchboard. If there is no plugin the jack J the operator inserts the calling plug P3 of the cord LCinto the jack J thereby disconnecting the line lamp circuit and applyingground potential to the sleeve conductor of the cordat the second point.

Signalling the called subscriber In order to start the signalling, orringing,

tors 303 and 3| The application of ground po-' tential to ring-startconductor 303 results in the closure of a circuit, through contacts ofthe operated talking key K4, for ring-start relay 2|2 in the local cordLC. Upon the closure of this circuit, ring-start relay 2|2 operates anddisconnects the upper winding of ring-cut-ofi relay 2 from the sleeveconductor of the cord, whereupon relay 2 restores and closes a lockingcircuit for relay 2|2 at its inner contacts to the grounded conductor25| by way of contacts of key K4 and contacts of relay 209 in multiple.This locking circuit serves to maintain the ring-start relay 2 I2operated after the rifig-startkey K5, Fig. 3, has i been restored.

At its lower armature ring-start relay 2| 2' opens the normal connectionbetween the ring conductor of the plug P3 and the correspondingconductor of the plug P2 and connects the ring conductor of the plug P3tothe ringing lead 2|8 by way of the lower winding of ring-cut-oif relay2| At the same time, relay 2|2 at its upper contacts places groundpotential on the tip conductor of the plug P3 so as to afl'ord alow-impedance return path for the ringing current.

The ringing current received over, the conductor 2| 0 is preferably, ofan intermittent nature,

The operator now restores talking key K4, .thereby disassociati'ng herequipment tromthe cord LC, and removing the shunt from around thearmature and front contact of the energized answering-bridge relay 209.The maintenance of the locking circuit of ring-start relay 2I2 toconductor I is thus rendered dependent upon the continued energizationof relay' 209, whereby the signalling of the called substation isimmediately terminated by the restoration of relay 2 I2 in the eventthat the calling subscriber repl'aces ,his receiver before the call isanswered.

When the subscriber at substation A responds by removing his receiver,theusual direct-cur rent bridge is closedacro'ss the conductors I02 andI03- of his line, closing a direct-current path for the lower winding'ofthe ring-cut-ofl relay 7 II from the (negative potential superimposed on.t 'e ringing currentreceived over conductor 2I0,

the circuit being completed over. the called line and to ground at theupper contacts of ring-start relay 2I2.

Since the continued energization of relay 2I2 nas been dependent uponthe locking circuit esestablished for relay 2I2 at the inner lowercontacts thereof following the restoration of the ring-start key K5,Fig. 3, the operation of relay 2 at this time results in the opening ofthe the plug P3 and jack J l and over the line of the called substationA. Relay 2I0 disconnects the calling supervisory lamp 2" at its lowercontacts, thereby extinguishing the lamp to give answered supervision tothe operator.

Ring-cut-ofi relay 2 is again energized over the sleeve conductor of theplugs P2- and P3 and through the inner upper contacts of ringstart relay2I2 to maintain the conductor 25I grounded at its lower contacts,preparatory to lighting the lamps 216 and 2H as disconnect signals. Fromthis it will be understood'that the relay 2 is a combined sleeve relayand ringcut-oif relay in that it controls the termination of the ringingperiod as above explained and also controls the application of groundpotential to conductor 25I for supervisory purposes.

Invasion signal It is is to be noted that the upper contacts of thecalling bridge relay2 I 0 apply ground potential to the normally opencontact pair of the talking key K4 with which tone-relay-conductor 304is associated. By this provision, if the operator attempts to invade thesecrecy of a conversation by throwing her talking, or listening, key K4after the called subscriber has responded and energized the callingbridge relay 2 I 0, ground po-. tential is extended from the contacts ofrelay 2l0 through the contacts of the key K4 to tonerelay conductor 304,thereby operating the tone relay 30l, Fig. 3, to apply an invasion-tonesignal. The application of the tone signal is made from the tone lead300 through contacts ,of relay 30I and resistor 32I to the upperleft-hand winding of the induction coil 10. A tone current is generatedin the secondary winding of the induction coil IC and is transmittedover the operator's tip and ring conductors and through contacts oi. the

' bridge relays 209 and 2I0 both restore, closing talking key to thetalking conductors of the cord circuit LC, thereby warning thesubscribers that their conversation is being listened to and alsowarning the operator that her headset conduc tors are telepho'nicallyconnected with a cord circuit over which a conversation is in progress,so that she may immediately restore the key K4 and thereby disconnecther headset conductors from the conversational path.

Disconnect supervision at the local cord Whenthe conversation has beenconcluded, the two subscribers replace their receivers, whereupon theanswering-bridge and callingcircuits for the supervisory relays 2I6 and2" from conductor 25I, grounded at the lower contacts of relay 2| I.These lamps now both light as disconnect signals tothe operator. In theevent that one subscriber delays in replacing his 20 receiver, thedisconnect signal associated with the other subscriber becomes lighted,andthe operator takes down the connectionin either case. When theconnection is taken down, the relay 2II restores, extinguishing lamps2I6 and 26 An incoming call The operations involved in extending anincoming call will now be considered. For this purpose it will beassumed that the trunk line TL is taken for use in the main exchange andthat ringing-current is 'projected over the conductors I38 and I39thereof in the usual manner. The upper winding of ring-up relay 2isnormally bridged across the trimk conductors I30 and I39 by way of thecontacts of the dial key K3, conductors I36 andv I31, contacts of thesplit relay H3, contacts of relays IIO, III, and H2, and the condenserI40. Ring-up relay II2 therefore responds to the ringing current, and atits lower contacts it closes a locking circuit for itself to groundthrough contacts of hold relay III and contacts of sleeve relay H2. Thelock- 'ing circuit of the lower winding of relay II2 includes line lampI2I and the common lamp The operator responds When the operator notesthe lighted condition of line lamp I2I, she throws her talking key K2 soas to get into telephonic communication with the calling subscriber. Thekey K2 connects the calling tip conductor of the trunk cord TC to theoperators headset tip conductor 3I5, and it connects the calling ringconductor to the operators headset ring conductor SIG. These conductorsextend through the cable C2 to the operators headset and positionequipment of Fig. 3, conductor 3I5 extending directly to the inductioncoil IC, while conductor SIG passes through the condenser 3" so as toavoid placing a direct-current bridge across the calling conductors ofthe trunk cord TC and a consequent spurious operation of line relay I00.

The key K2 also associates the common ringstart conductor 3| I with thering-start relay I00; connects the common split conductor 3I2 to theupper winding of split relay I I3; places a ground potential on thetransmitter-ground conductor III to energize the operator's transmitter;and it closes a circuit from the grounded conductor I5I for hold relayIII, conductor I5I being grounded through contacts oi! line relay I08and contacts of key KI. Hold relay III operates and locks itself toconductor I5I, whereby relay III, having once been operated, remainsoperated either until line relay I 08 energizes or until the key KI ismomentarily operated as a release or clear-out key. It is to be notedthat the hold lamp I20 is lighted in parallel with the winding of holdrelay III to serve as a signal to the operator that an answered butundisposed-oi incoming call has been received over the trunk cord TC.

Giving answered supermsion to the main exchange At its inner uppercontacts, hold relay I I I completes the normally open talking circuitto the right-hand windings of repeating coil II9, by joining conductorsI33 and I35, thereby bridging the right-hand windings oi! repeating coilI09 across the conductors I38 and I39 of the trunk line TL to giveanswered supervision to the main exchange and to bring about a cessationin the applied ringing current. Hold relay III also opens the ring-upbridge at another point at its upper armature, while at its inner lowerarmature it opens the locking circuit of ring-up relay I I2 to restorethe ring-up relay and to extinguish the line lamp I2I.

The operator now converses with the calling subscriber and securesinformation concerning how the call is to be completed. In order toexplain the call-oflering feature of the trunk cord circuit, it will beassumed that the calling subscriber is requesting certain informationand is not asking for a connection to a specific local line. It may befurther assumed that the operator does not know for a certainty which ofthe various local subscribers has the desired information. In this eventthe operator proceeds to get in touch with local subscribers one afteranother until she discovers the right one. It is thought best that thecalling subscriber be rendered unable to hear the conversation takingplace between the operator and the successive local subscribers, forwhich reason cord splitting facilities have been provided, operable atthe discretion of the operator.

Splitting the cord TC In order to split the trunk cord TC, the operator,while still maintaining the talking key K2 of the cord TC operated,operatesw the split key K6, 9. locking key. As long as the split key K8remains operated, ground potential is maintained on split conductor 3I2,closing a circuit through contacts of the talking key K2 for the upperwinding of split relay II3. Split relay II3 closes an answeringsupervisory bridge across conductors I36 and I3! at its lower armature,thereby maintaining a state of answered supervision on the trunk lineTL. This relay also disconnects talking conductors I36 and I31 fromtalking conductors I35 and I34 at the normally closed contactscontrolled by the upper and lower armatures respectively of the relay.The trunk line TL is thus telephonically disconnected or split from thetrunk cord TC.

Inquiry call to a restricted line The operator may now proceed to makean inquiry connection with a local line, the line of substation C, Fig.2, for example. She does this by inserting the plug PI into the jack J3,thereby clearing the line conductors 202 and 203 of the ground and linelamp connection. Ground potential on the sleeve of the jack J energizesthe sleeve conductor I52 of the cord TC, closing a circuit throughcontacts of line relay I08 for the calling supervisory lamp II9, by wayof contacts of the night key KI and lamp conductor 4I2. At the sametime, a circuit is closed from the grounded sleeve conductor I52 throughjumper I53 for the marginal sleeve relay 'I I0. Sleeve relay IIO,however, does not operate at this time because the jack J3 has groundpotential connected to the sleeve terminal thereof through a resistor201.

'Having inserted the plug PI into the jack J2 of the line of substationB, Fig. 2, the operator momentarily operates her ring-start key K5, Fig.3, momentarily applying ground potential to ring-start conductors 3H and303. The application of ground potential to ring-start conductor 3closes a circuit through contacts of the talking key K2 of the trunkcord TC for ring-start relay I05. Relay I05 closes a locking circuit foritself by way of contacts of ring-cut-oflf relay I06 to conductor I5I,grounded through contacts of line relay I08 and contacts of thenight-key KI.

Ringing currentfrom lead I25 is now applied through the operated lowerarmature of relay I05 to the ring conductor of the called line,returning to ground over the tip conductor through the upper contacts ofrelay I05.

When the subscriber at substation C responds, ring-cut-oif-relay I06 isoperated in a directcurrent circuit including the called line, and itunlocks ring-start relay I05 and permits it to restore. The ringingoperation is thereby terminated. Line relay I08 is now operated over thecalled line and it disconnects supervisory lamp II9 from sleeveconductor I52, thereby extinguishing the supervisory lamp II 9 to giveanswering supervision. At the same time, relay I08, at its middlearmature breaks one point in the ground connection to conductor I5I todeenergize the hold relay I I I, but the hold relay II I remainsoperated for conductor I5I is maintained grounded through the innerlower contacts of the operated split relay H3. Line relay I08, at itslower contacts, closes a point in the circuit for giving theinvasion-tone signal, indicative oi. the fact that the operator islistening in on the connection, but this circuit does not becomeeffective at this time because of the operated condition of split relayII3, which relay, when operated, precludes conversation between thecalled local subscriber and anyone else but the operator.

The operator and the subscriber at substation B now converse. It will beassumed that the subscriber at substation C informs the operator thatthe subscriber at substation A is the one who should give theinformation to the calling subscriber. That being the case, the operatorremoves the plug PI from the jack J2 and inserts it into the jack JI.When the plug is removed, relay I08 restores.

Extending the incoming call Relay IIO operates when the plug is insertedinto the jack J I, as the sleeve of this jack is directly grounded.Following the insertion of the plug PI into the jack J I, the operatormomentarily operates her ring-start key K5, to again operate thering-start relay I05 to start the signalling of substation A. At thispoint, the operator restores the split key K5 in her position equipment,thereby permitting split relay II3 in the trunk cord TC to restore andreconnect the formerly disconnected sections of the talking circuit.Also the operator restores the talking key K2, as she has no occasion toconverse with the subscriber at substation A when he replies.

Local answered supervision When the subscriber as substation A responds,ring-cut-oif relay I06 operates as before and unlocks ring-start relayI05, whereupon relay I05 restores and terminates the ringing operation.Line relay I08 now operates over the line of substation A. At this time,since the split relay H3 is not operated line relay I08 removes theground potential normally applied to conductor I5I through contacts ofthe key KI, opening the circuit of the locked hold relay III andpermitting this relay to restore, and the hold lamp I20 to becomeextinguished. With the relay III r estored, the continued state ofanswered supervision is dependent upon the continued operation of sleeverelay IIO, as the talking connection between conductors I33 and I35 isnow completed only at the upper contacts of relay IIO. As before, linerelay I08 disconnects supervisory lamp H9, thereby extinguishing it togive answered supervision.

The subscriber at substation A may now converse over the trunk line TLwith the calling subscriber.

Invasion signal It may be pointed out that in the event that theoperator (inadvertently or otherwise) operates the talking key K2- whilethe subscriber at substation A is conversing with the calling subscriberby way of the trunk line TL, the contacts of the key K2 connecttone-relay conductor 3I3 to ground through contacts of the operated linerelay H8 and contacts of split relay II3. This brings about an operationof tone relay 302 to cause the warning tone to be applied ashereinbefore described, thereby warning the operator and the subscribersthat the operator has invaded the secrecy of the conversation.

A temporary transfer connection For the purpose of explaining the recalland temporary transfer feature of the private branch exchange, it willbe assumed now that the subscriber at substation A is unable to supply acertain feature of information desired by the calling main exchangesubscriber. Under this condition, the subscriber at substation Arequests the main exchange subscriber to hold the line for a mo mentwhile he secures the desired information from someone else. As the firststep in securing this information, the subscriber at substation Amomentarily depresses the recall push button IOI, temporarily groundingconductor 102 of his line. This operation unbalances the current flow inthe windings of the differential recall relay I01, causing the relay tooperate.

Upon operating, relay I01 closes a circuit through contacts of splitrelay II3 for control relay I I4, through the resistor II 5 to thenegative pole of the exchange battery. Control relay Ill locks itself atits lower contacts to ground through the lower winding of split relay H3and contacts of the operated sleeve relay II 0. But relay II3 does notoperate in this locking circuit until the recall button IOI is releasedand recall relay I01 is restored to open the initial circuit of relay II4.

Upon operating in series with relay H4, relay 6 H3 prepares a temporarylocking circuit for its upper winding by way of the contacts of recallrelay I01; disconnects the armature of recall relay I01 from theright-hand terminal of relay Ill and connects it to the Junction of thewinding of relay III and resistor II5, preparatory to restoring relayH4; and at its upper and lower armatures and associated contacts itdisconnects the trunk line conductors I36 and I31 from the conductorsI35 and I3, at the same time removing the shunt normally existing aroundthe tip and ring contacts of the jack J0 and connecting conductors I34and I35 to the tip and ring contacts of the jack J4.

The conductors I36 and I31 are bridged by the 20 lower armature of relayH3 and its associated contact, thereby holding a state of answeredsupervision on the conductors I30 and I31 and consequently on conductorsI38 and I39 of the trunk line TL, as regards the main exchangeequipment.

The recall lamp I I6 is lighted through contacts of'control relay H4 andcontacts of the jack J4 to signal the operator. The operator responds tothis signal by inserting the answering plug of an idle cord circuit, theanswering plug P2 of the cord circuit LC, for example, into the jack J4.The lamp H6 is thereupon extinguished by the opening of the cut-oilcontacts of the jack. Answering-bridge relay 209 is energized throughcontacts of jack J4 and the right-hand winding of repeating coil I09 tomaintain answering supervisory lamp 2I5 unlighted. The operator, bythrowing the talking key K4, converses with the subscriber at substationA, the calling main-exchange sumcriber being unable to hear theconversation.

Upon learning who the subscriber at substation A desires to conversewith, the subscriber at substation B for example, the operator insertsthe calling plug P3 of the cord circuit LC into the jack J2, completingthe connection between the line of substation A and the line ofsubstation B. The operator causes the subscriber at substation B to besignalled in the hereinbefore described manner.

When the subscriber at substation B responds, the talking connection iscompleted through the local cord LC from the line of substation Cthrough the jack J 4 to the line of substation A, so that the two localsubscribers may converse as desired.

Temporary transfer back to main exchange connection It maybe assumed nowthat the subscriber at 60 substation A, upon conversing with asubscriber at substation B, discovers that he needs to secure furtherdata from the calling subscriber before the subscriber at substation Bcan give the complete information. That being the case, 65 thesubscriber at substation A requests the subscriber at substation B tohold the line for a moment while he again converses with the callingmain-exchange subscriber.

T0 transfer the connection back to the trunk 70 line TL, the subscriberat substation A again momentarily operates his recall button IOI. Recallrelay I01 is again momentarily operated to apply ground potential to themiddle upper armature of relay I I3. At this time, a temporary lock.

' calling main exchange subscriber, he may again,

ing circuit for the upper winding of split relay H5 is closed at theinner upper contacts of the relay, and a shunt circuit for relay iiiandthelower winding oi relay H3 is closed through the middleupperarmature and working contact of relay H3. The resistor i 05prevents the flow of excessive current in this shunt circuit. Relay litrestores and opens its locking circuit at its lower contacts.

When the recall button i ti is restored and the back to the trunk lineTL. It is to be noted that 1 the ring terminal of the jack J4 is nowconnected to the tip terminal of the jack through the upper armature andresting contact of relay H3, hold-= Transfer back to the localconnection As soon as the subscriber at substation A has received thedesired additional data from the relay II4 when recall button IOI isrestored and recall relay IIl'I restores.

with the connection again transferred from the trunk line TL to the lineof substation B by way of the local cord LC and the jack J4, thesubscriber at substation A again converses with the subscriber atsubstation B pursuant to obtaining .the desired information. I

Clearing out the transfer connection It may be assumed now that thesubscriber at substation A issatisfied with the information he receivesfrom the subscriber at substation B and" the conversation between thetwo subscribers is thereby terminated. Under this condition, thesubscriber at substation A again momentarily operates his recall buttonIIJI, bringing'about the successive restoration of control and splitrelays H4 and H3 ofthe trunkcord TC. At the same time, the subscriber atsubstation Breplaces his receiver (not shown) whereupon disconnectsupervision is given by way of the. calling supervisory lamp 2" becauseof the restoration of the calling-bridge relay 2III. The operatortherefore removes the plug P2 from the jack J 4 and removes When thecalling subscriber at substation A replaces his receiver at the end ofthe conversation with the main-exchange subscriber, line relay I08restores and closes a circuit for supervisory lamp W H9 through contactsof the key Ki and lamp conductor M2, to light the lamp H9 as adisconnect signal. 1

M9, the operator may either take down the con- 4 nection immediately bywithdrawing the plug Pi from the jack 58, or she may again throw hertalking key Kl to inquire of the calling main exchange subscriberwhether he has completed his transaction with the establishmentowningthe private branch exchange. The operator follows one procedure orthe other, depending entirely upon her instructions. ceeds in the lastmentioned way, the operation of the key K2 reoperates holding relay I lland relights hold lamp I20. Relay III shunts the upper contacts ofsleeve relay IIIJ, permitting the operator to withdraw theplug Plfrom-the jack J1 and transfer the incoming connection to any other line.

b At any rate, when the plug PI is finally removed from the jack lastused (hold relay I01 has been restored at such time) sleeve relay IIOrestores and disconnects conductors I35 and I33, thereby removing theright-hand windings of repeating coil I09 from in bridge of theconductors of the trunk line TL and giving disconnect supervision in themain exchange. 1

Outqoing call from the branch exchange In the event that shepro- The wayin which outgoing service from the branch exchange is handled will nowbe explained. For this purpose it is assumed that the subscriber atsubstation A, desiring to make .a call to the main exchange, removes hisreceiver and thereby causes the line lamp I84 associated with the jack JI to become lighted as a calling signal. The operator responds to thissignal by inserting the answering, plug of a local cord, such as theanswering plug P2 of the local cord LC, into the jack J I, answering thecall in the manner hereinbefore described.

I Upon conversing with the subscriber at substation A and learning thathe desires to make a -main exchange call, the operator removes the plugP2 from the jack J I and inserts the plug of an idle trunk cord, theplug PI of the trunk cord T0 for example, into the jack J I. Theoperator does not thereafter need to pay further heed to the outgoingconnection in the event that the main exchange is a manual exchange, forthe act of inserting the plug PI into the jack J I signals the mainexchange operator, as will now be pointed out. Since the receiver is offat the substation A, line relay I08 is energized in the usual way tomaintain the. supervisory lamp II9 exv tinguished. Sleeve relay. I IIIis energized over sleeve conductor I52 and at its upper contacts itplaces the right-hand windings of. repeating coil I09 in bridge of theconductors of the trunk line TL, closing a direct current circuit overtrunk line TL to bring in the usual line signal in the main exchange. I

' When the conversation has been completed, and the subscriber atsubstation A has replaced his receiver, the restoration of line relayI08 results in the lighting of supervisory lamp -I I9 as a disconnectsignal, whereupon the operator may take down the connection.

Withholding main exchange service from re-- striated lines In order toconfine the inter-exchange traific more closely to oflicial businesscalls, it is customary for the management of an organization using aprivate branch exchange to specify that certain lines shall not bepermitted switching service as regards main exchange connections.Provisions have been made in the private branch exchange disclosedherein for enforcing this rule notwithstanding possible collusionbetween a restricted-line subscriber and the branch exchange operator.The way in which this rule is enforced will now be pointed out moreparticularly.

The ground connection to the sleeve terminal of the jack of eachrestricted line is made through a resistor such as theresistor 20'Iassociated with the jack J3 of the restricted substation C, Thisresistor has been so chosen and the local cords LC have been so designedthat the restricting resistors do not interfere with the putting up oflocal connections, but the putting up of a main exchange connection to arestricted'line is prevented. It is to be noted that the sleeveterminals on the plugs P2 and P3 of the local cord LC are joined by asleeve conductor, and that over this conductor the relatively highresistance winding of ring-cut-oflf relay 2 is controlled in the mannerhereinbefore explained to control the cutting off of the ringing currentand the lighting'of supervisory lamps 2I6 and 2H. Relay 2II operatessatisfactorily through a resistor such as 201. On the other hand, thesleeve relay IIO of the trunk cord TC, together with the similar sleeverelays of the other trunk cords, is so wound and adjusted that the relaydoes not operate in series with a resistor such as 201. Keeping in mindthat sleeve relay IIO must be in an energized condition to maintainconductors I33 and I35 joined together after hold relay III has fallenback, it will be seen that no conversation can take place over the trunkline TL and through the plug PI when the sleeve current obtained from ajack is insuflicient to operate the marginally adjusted sleeve relay II0. It will be recalled that conductors I33 and I35 are initially joinedby the hold relay III when 'relay' III operates responsive to theinitial manipulation of talking key K2. But relay III is restored by theoperation of line relay I00 when the local subscriber answers, unlessthe operator is making an inquiry call, as hereinbefore explained, inwhich case split relay II3 prevents the restoration of relay III, butalso disconnects the main exchange trunk.

From this it will be understood that the incoming trunk is opened anddisconnect supervision is given to the main exchange operator when thelocal restricted service subscriber removes his receiver to answer'thecall which has been negligently or otherwise extended to his line by thebranch exchange operator. The trunk is thus freed for further use at themain exchange and- Preventing barred conversation over operator'slistening taps It is to be noted that disconnect relay 302, in additionto energizing the operators transmitter, disconnects the local tip andring listening and talking conductors 305 and 306 from the operatorsheadset conductors and consequently from the corresponding conductorsassociated with the trunk cords such as TC. This arrangement insuresthat the operator cannot surreptitiously complete a connection between arestricted service line and a trunk line such as TL over her headsetconductors through the use of a local cord plugged into the restrictedservice line (with the talking key thereon thrown) and a trunk cordplugged into a non-restricted line (with the talking key such as K2thrown thereon),

.as the disconnect relay 302 under such conditions severs thelistening-lead connection between the local cord and the trunk cord.

The foregoing provisions bar restricted lines also from outgoing trunkcalls.

matic exchange When the main exchange is an automatic exchange, thebranch-exchange operator is required to send the impulses necessary toposition the switches in the automatic exchange. She does this bythrowing the dial key K3 to transfer conductors I I38 and I33 from theholding bridge through the right-hand windings of repeating coil I09into association with the calling device CD, which calling device iscommon to all of the trunk lines such as TL. This calling device isoperated in the usual manner to transmit the series of impulses, orinterruptions, being provided with a condenser II! and resistor II8which cooperate to reduce sparking and im-v prove the quality of theimpulses. For operation in conjunction with a manual main exchange, thecalling device CD is omitted, in which case the dial key K3 may be usedas a flash" key, to flash or intermittently signal the mainexchangeoperator when it is desired to attract her attention during the processof setting up or receiving calls.

Through supervision from a local line to the main exchange sociated withthe sleeve relay H0, in which case the connection between sleeveconductor I52 and sleeve relay IIO is controlled by line relay I03.Keeping in mind that the supervisory bridge through the right-handwindings of repeating coil I09 is controlled at the upper contacts ofsleeve relay I I0 it will be seen that the immediate restoration ofsleeve relay IIO following the restoration of line relay I00 gives theusual disconnect supervision in the main exchange. Line relay I08 isprovided with a retarding sleeve, as indicated in the drawings, so as torender the relay slightly slow releasing to guard against falsedisconnect signals occasioned by inadvertent momentary manipulation ofthe hookswitch at the local telephone. With the jumper I53 removed, theoperations involved in setting up connections through the trunk cord TCare the same as described hereinbefore, except that the inter-exchangetrunk holding circuit is opened immediately upon the receiver beingreplaced at the local substation, as above noted,

In connection with the feature being discussed, it may be pointed outthat the local operator is preferably instructed to remove the plug PIfrom the local jack immediately upon receiving the disconnect signal H9and without challenging the connection or making inquiries, when thethrough-supervision feature is used, thereby greatly speeding up theservice and minimizing the work of the branch exchange operator. It willbe recalled, of course, that the recall jack J4 and lamp I I6 areprovided to enable those subscribers having access to main exchangeservice to recall the local operator without relying on "flashing" thehookswitch. Therefore, if the main exchange connection is to betransferred to another local line, the local subscriber involved may getin touch with the operator by the recall operation hereinbefore pointedout rather than by a flashing of the hookswitch, thereby removing allnecessity for the local operator to challenge a disconnect signal beforeshe takes down the connection.

Night service at the branch exchange In order to provide night servicefor such employees of the organization served by the branch exchange asmay be on duty at times other than the regular hours for switchboardoperation, the trunk cords such as TC are arranged so that one or moremay be plugged into local jacks to give direct service to and from themain exchange. For example, if the line of substation A is to be usedduring the time when the regular operator is not in attendance at thebranch exchange, the plug PI may be inserted into the jack J I, with thenight key KI operated. The night key disconnects the supervisory lamp 0as a current saving feature, and it disconnects the windings ofrepeating coil I09 from in bridge of the two sections respectively ofthe talking circuit and joins conductors I3I and I32 directly with theconductors I33 and I34, connecting the tip and ring conductors of theplug PI straight through to the conductors I38 and I39 of the trunk lineTL, by way of contacts of sleeve relay H0. The inner lower pair ofcontacts of the night key KI, shunts the terminals across which jumperI53 is connected, so that sleeve relay H0 is connected to the sleeve ofthe plug PI with the night key thrown, whether the jumper I53 is inplace or not. The sleeve relay H0 is therefore in operated position andmaintains conductors I33 and I closed so that ringing current receivedfrom the main exchange passes through to the local substation. At thispoint it may be mentioned that night service cannot be given to arestricted service line, such as the line of substation C, Fig. 2,because sleeve relay IIO will not operate through a resistor such as 201to join conductors E33 and I35 together.

In the event that the main exchange is an automatic exchange, the locallines plugged up as above for direct night service with the mainexchange may be automatic telephones equipped with the usual callingdevice. This calling device is used in the usual way to call mainexchange numbers.

The night alarm system whole time to switchboard operation, it isadvantageous to provide a suitable audible alarm circuit to call theattention of the night operator when such attention is needed. The nightalarm circuit of Fig. 4 has been provided to give a satisfactory serviceof this nature in an economical manner.

Previous arrangements for giving night service have made use of a seriesrelay (such as series relay 403 in local lead M2) in a line-lamp leadsuch as M I, but such a relay requires frequent adjustment when a largenumber of lines are connected to the branch exchange because of theeffect of shunts or line leaks, which are usually negligible as far as asingle line is concerned, but which become considerable when a largenumber of lines are involved. The alarm circuit of Fig. 4 responds to achange of current and is largely independent of the amount of currentflowing over conductor 4. v

In order to place the night alarm circuit in receptive condition, thenight-alarm key K1 is operated. The upper contacts of the key Kl removethe shunt from around the primary wind- I ing of the induction coil 400;the lower contacts of the key remove the shunt from around series relay403, and apply energizing potential to the priming winding of theline-pulse relay 40 I; while the middle contacts of the key, arranged tomake last, connect the secondary winding of induction coil 400 in serieswith the operating winding of relay 40I through contacts of auxiliaryrelay 402. Relay 40I is so designed and adjusted that the current-flowthrough the lower winding, the priming winding, energizes the relaysuficiently to render it near the verge of operating, but notsufficiently to operate the relay. The relay is therefore in a verysensitive condition, awaiting an impulse at the induction coil 400. Anyimpulse that occurs by the removal of the shunt from around the primarywinding of the induction coil 400 at the upper contacts of the key hasbeen dissipated by the time secondary winding of the coil and the upperwinding of relay 40I have been connected in the circuit.

With the key Kl operated, the removal of the receiver at a substation,such as B or C, closes a circuit for the line lamp, such as 204 or 208,over conductor 4 in series with the primary winding of induction coil400. Such current increase change as occurs takes place rather rapidlybecause of the relatively large amount of current drawn by the filamentof a lamp when the lamp is cold (a tungsten or similar filament beingpreferred). The rise of current in the primary winding causes an inducedcurrent to flow in the secondary winding of induction coil 400 throughthe contacts of the key K1, the line Winding of relay 4M, and contactsof relay 402. The direction of this current is such as to assist themagnetic field already set up by the energization of the priming windingof relay 40I, causing relay 40I to operate. The impulse is soondissipated, but relay 40I, having once operated, remains operated fromthe energization produced by the priming winding, this tendency beingwell known.

Auxiliary relay 402 operates; it first disconnects the line winding ofrelay 40I to prevent a reverse impulse therethrough; at its middlearmature, relay 402 closes a shunt around the primary winding of coil400 so as to afford a direct lighting circuit for the line lamp at thecalling station; and at its upper armature it closes a circuit throughthe windings of retard coil 404 for the buzzer B. The buzzer B isshunted by the condenser 406, which condenser operates in con- Junctionwith the retard coil 404 to avoid ripples in the potential of the branchexchange battery occasioned by the action of the buzzer.

The circuits of Fig. 4 remain in this condition, with buzzer 405operating until the night operator responds to the signal and restoresthe key K'I.

When the key K1 is restored, the circuit of the lower winding of relay404 is opened, causing relay 40I to restore and bring about therestoration of auxiliary relay 402, and the consequent cessation of thebuzzer signal.

After the night operator has disposed 01 the call, he reoperates thealarm key K! to position the night alarm circuit for further operation.

With the night alarm key K1 in operated position, series relay 403responds to a flow of current over the local lamp conductor 2 in theevent that the circuit of a supervisory lamp such as 2 I6 or 2H, Fig. 2,or the circuit of a line or supervisory lamp such as I2! or H9, Fig. 1,is closed. Relay 403 operates the buzzer 405 attracting the attention ofthe night operator.

Having described the invention, what is considered to be new and isdesired to be protected by Letters Patent will be set forth in thefollowing claims.

What is claimed is:

1. In a private branch exchange served by a manual switchboard andtrunk-connected with a main exchange, a trunk cord for interconnecting amain-exchange trunk with local lines, local cords for interconnectinglocal lines with each other, and means effective when the main exchangetrunk line is connected through the trunk cord to a local line forsetting up a connection by way of a local cord and through the trunkcord between such local line and another local line.

2. In a private branch exchange served by a manual switchboard andtrunk-connected with a main exchange, a trunk cord for interconnecting amain-exchange trunk with local lines, local cords for interconnectinglocal lines with each other, means effective when the main exchangetrunk line is connected through the trunk cord to a local line forsetting up a connection by way of a local cord and through the trunkcord between such local line and another local line, and meanscontrolled from the substation on the trunk-connected local line andoperable at the will of the subscriber on such line for transferring thetelephonic connection of his line from the trunk line to the said otherlocal line through said local cord circuit and for transferring theconnection back again from the said other local line to the trunk line,as often as may be desired.

3. In a telephone system including a manual switchboard, a cord circuitthrough which connections may be extended to and from subscribers lines,means in a cord circuit controlled by the fiow of transmitter current toa. connected line for giving answering and disconnect supervision, arecall device in the cord circuit nonresponsive to control over the linewhen answering said disconnect supervision is given, means controlled bya subscriber on a line connected with by said cord circuit for placing aspecial condition on the line to bring about the operation of the saidrecall device, and means for signalling the operator when the recalldevice is operated.

4. In a telephone system served by a manual switchboard, a cord circuitfor setting up connections between lines, a supply circuit at the cordcircuit for supplying transmitter current to a connected line, adifferential relay in said supply circuit at the cord circuitnon-responsive to the normal balanced condition of the transmittersupply circuit, means operable at the will of the subscriber on aconnected line for unbalanclfig his line to bring about the operation orsaid differential relay, and an operator's signal controlled by saiddifferential relay to inform the operator when the subscriber causes thediflerential relay to be operated.

5. In a telephone exchange served by a manual switchboard, a cordcircuit for use in interconnecting the lines, a transmitter-currentsupply connection in said cord circuit effective to supply transmittercurrent to a connected line, a disconnect relay and a recall relayconnected in said transmitter supply circuit, and means controlled bythe subscriber on a connected line for selectively controlling the saidrelays.

6. In a private branch exchange served by a manual switchboardtrunk-connected to the main exchange, a trunk cord and plug at themanual switchboard terminating a trunk line incoming from the mainexchange, a hold signal and means for displaying it responsive to theoperator having answered an incoming call, and means for maintainingsaid hold signal displayed until the operator has extended the incomingconnection to a local line.

7. In a private branch exchange served by a manual switchboardtrunk-connected with the main exchange, a talking key associated with amain exchange trunk and operable to place the operator in telephoniccommunication with the trunk to answer an incoming call, a hold signalassociated with the trunk and means for displaying it responsive to theoperation 01' the talking key, and means for maintaining the hold signaldisplayed after the talking key has been restored and until the incomingconnection has been extended to a local line.

8. In a private branch exchange served by a manual switchboardtrunk-connected with the main exchange, a talking key at a trunk lineoperable by the branch exchange operator to enable her to answer anincoming call, locking means controlled by operation of the talking keyto place and maintain a state of answered supervision on the incomingtrunk line when the incoming call is answered, means eflective alter theoperator has extended the incoming connection to a local line and thesubscriber thereon has answered for restoring the locking means, meansincluding a supervisory relay controlled over the called line formaintaining the state 01 answered supervision on the incoming trunk lineduring the conversation between the main exchange subscriber and thelocal subscriber, and means controlled by the restoration of saidsupervisory relay when the local subscriber replaces his receiver fortransferring the incoming trunk line into a state of disconnectsupervision and for giving disconnect supervision to the branch exchangeoperator.

9. In a telephone system served by a manual switchboard, cord circuitsat said switchboard for completing connections from calling lines tocalled lines, operator's telephone conductors, a talking key at eachcord circuit for connecting the operator's telephone conductors to thetalking conductors of a cord circuit to enable the operator to conversewith the calling subscriber, a supervisory relay in the cord circuit andmeans for operating it when the called subscriber answers the call, andmeans controlled Jointly by switchboard, cord circuits through whichconnections are extended to called lines. a talking key at each'cordcircuit operable to place the operator into telephonic communicationwith a subscriber by way of the cord circuit, a supervisory relay ineach cord circuit operated when the called subscriber answers, meanscontrolled jointly by the said supervisory relay and the talka ing keyfor applying a distinctive audible frequency tone to the talkingconductors of the cord in the event that the talking key is operatedduring the progress of a conversation through the cord circuit, cordsplitting means in the cord circuit operable to enable the operator todivide the cord into two sections and converse with the calledsubscriber without being overheard by the calling subscriber, and meansfor preventing the said application of distinctive tone current.

while said splitting means is operated.

11. In a telephone system in which connections are set up through cordplugs and line jacks, local cord plugs and a trunk cord plug, asensitive sleeve relay associated with each, local cord plug, a marginalsleeve relay associated with the trunk cord plug, circuit connectionswith certain of the local Jacks for enabling either a sensitive or amarginal sleeve relay to energize thereover, and circuit connectionsassociated with the sleeve conductors of certain other jacks over whichthe sensitive sleeve relays of the local cords may operate. but overwhich the marginal sleeve relay oi the trunk cord will not operate, andmeans in the trunk cord for completing an eiiective talking connectiontherethrough controlled by the sleeve relay, whereby any, local line maybe connected telephonically with any other local line and a talkingtrunk connection may be set up be-' tween the trunk and certain of saidlines and not between the trunk and certain of the other lines.

12. In a manual switchboard used to give local service and trunk serviceto subscribers lines,

certain 01 said lines being restricted to local service while otherlines are permittedtrunk service, trunk cords for setting up trunkconnections to lines entitled thereto, local cords for setting up localconnections, operator's headset conductors, s a separate talking key ateach of the cord circuits for connecting the operator's headsetconductors to the talking conductors oi the cord to enable .the operatorto converse with the calling subscriber, and means eflective wheneverthe talking key 01' a trunk cord is thrown for disconnecting theoperator's leads from the local cords, whereby a restricted line may notbe connected with a trunk line through a local cord and a trunk cord inthe event that a local-cord talking key and a trtgnk-cord talking keyare simultaneously opera d. v v

13. In a branch-exchange telephone system, trunk lines connecting thebranch exchange with the main exchange for giving main-exchange 20)service to branch exchange subscribers, a switchboard cord and plugterminating a main-exchange trunk, a normally open point in one ofthetalking conductors of said trunk. a supervisory relay controlledthrough the cord and plug and. over a local line to close said normallyopen point during conversation to give supervision to the main,exchange, and means associated with said cord and including a manuallyoperable key for bypassing said normally open point and for transierringthe trunk conductors directly through the plug to the connected line,whereby ringing current may be received directly by a local line fromthe main exchange and supervision may be directly given from the localline to the main exchange.

14. In a private branch exchange, means for extending connectionsbetween subscribers in the branch exchange, trunk lines to a mainexchange, means for extending connections from and to the subscriberslines in the branch exchange over the trunk lines, means in the trunkline operated by a calling or a called subscriber in the branch exchangefor switching the connection from the trunk line to a local connectingmeans, and for (El thereafter switching the connection back to the trunkline at will.

THOMAS F. CROCKER,

